Optical device

ABSTRACT

An optical device includes a light-emitting element for irradiating light onto an information recording medium, a diffraction grating for splitting light emitted from said light-emitting element into a plurality of beams, a focussing member for focussing the plurality of beams onto the information recording medium, a deflection member for deflecting the plurality of beams after they have been reflected from the information recording medium; and a photodetector for receiving the plurality of beams after they have been deflected by the deflection member. The diffraction grating has a first grating region and a second grating region, which have different diffraction efficiencies. The zero-order diffraction light in the first grating region is used as the main beam for reproducing the information signal, and the +1-order or −1-order diffraction light in the second grating regions is used as sub-beams for reproduction of the tracking error signal. Thus, the light amount of both the main beam and the sub-beams can be increased without increasing the light emission of the semiconductor laser element  1,  and the S/N ratio of the main beam and the sub-beams can be enhanced.

BACKGROUND OF THE INVENTION

[0001] 1. Field of the Invention

[0002] The present invention relates to an optical device.

[0003] 2. Description of the Prior Art

[0004] First of all, the configuration and operation of a conventionaloptical device are explained with reference to the FIGS. 13 to 16.

[0005]FIG. 13 is a drawing showing the optical system of a conventionaloptical device. As shown in FIG. 13, a diffraction grating 2 forproducing three light beams, a holographic element 3 serving as adeflection member, a collimator lens 4 serving as a focussing member,and an objective lens 5 serving as a focussing member are arranged inthat order on the optical axis of a light beam emitted from asemiconductor laser element 1 serving as a light-emitting element.

[0006] An information recording medium 6 is arranged in the focussingplane of the objective lens 5. Photodetector groups 7 with a pluralityof photodetectors for receiving light that has been deflected by theholographic element 3 are arranged on both sides of the semiconductorlaser element 1.

[0007] As shown in FIG. 14, a parallel grating with constant pitchextending in the X-axis is formed on the diffraction grating 2.Moreover, a diffraction grating (not shown in the drawing) functioningas a lens is formed on the holographic element 3.

[0008] The following is an explanation of the propagation of lightemitted from the semiconductor laser element 1. As shown in FIG. 13,when passing the diffracting grating 2, the light emitted from thesemiconductor laser element 1 is diffracted by the diffraction grating 2in the direction of the Y-axis, and split into three light bundles,namely a +1-order diffraction beam, a −1-order diffraction beam and azero-order diffraction beam. It should be noted that, since the +1-orderdiffraction beam and the −1-order diffraction beam are diffracted in theY-axis direction, that is in directions perpendicular to the plane ofFIG. 13, they cannot be portrayed in distinction to the zero-orderdiffraction beam in this drawing. The zero-order diffraction beam isalso referred to as the main beam, and is used to obtain the signalrecorded on the information recording medium 6 and the focus errorsignal regarding the focus error between the optical device and theinformation recording medium 6. The ±1-order diffraction beams arereferred to as sub-beams, and are used to obtain the tracking errorsignal. After passing the holographic element 3, these three lightbundles are irradiated onto the collimator lens 4. The collimator lens 4collimates the light beams into parallel light, and the objective lens 5focuses the collimated light onto the information recording medium 6,where it is reflected back towards the objective lens 5.

[0009] Then, the light reflected by the information recording medium 6propagates along the same path in the opposite direction, that is,through the objective lens 5, the collimator lens 4, and the holographicelement 3, in that order. The light beams irradiated (again) onto theholographic element 3 are deflected in the X-axis direction, and enterthe photodetector groups 7. The photodetector groups 7 receive the mainbeam and the sub-beams, and a calculating element (not shown in thedrawings), which is connected to the photodetector group 7, calculatesthe signal recorded in the information recording medium 6, the focuserror signal, and the tracking error signal.

[0010] The zero-order diffraction light in the region 11 of FIG. 14enters the collimator lens 4, the +1-order diffraction light in theregion 12 enters the collimator lens 4, and the −1-order diffractionlight in the region 13 enters the collimator lens 4.

[0011] In this conventional optical device, the diffraction efficiencycan be set to an appropriate value by adjusting the diffraction gratingdepth of the diffraction grating 2. Here, “diffraction grating depth”means the extent of the spatial modulation of the diffraction grating,and for a refractive index-type diffraction grating for example, itmeans the size of the spatial modulation of the refractive index.

[0012]FIG. 15 illustrates the diffraction efficiency of zero-orderdiffraction light as a function of the grating depth of the diffractionelement 2 (line X) and the diffraction efficiency of ±1-orderdiffraction light as a function of the grating depth of the diffractionelement 2 (line Y). As becomes clear from FIG. 15, an increase of thediffraction efficiency of ±1-order diffraction light leads to a decreaseof the diffraction efficiency of zero-order diffraction light. This is adirect consequence of the law of the energy conservation.

[0013] Consequently, it was not possible to increase the light amountfor the main beam and the sub-beams and to enhance the S/N ratio forboth.

[0014] Moreover, conventional optical devices have the drawback that atilt in the track direction of the information recording medium 6 causesan offset for the tracking error signal obtained by differentialcalculation from the sub-beam spots on the information recording medium6. Such an offset may be caused by multiple reflections between the endface of the semiconductor laser element 1, the diffraction grating 2,the holographic element 3, and the information recording medium 6.

[0015] This mechanism is explained referring to the example shown inFIG. 16. The orientations of the X-axis, the Y-axis and the Z-axis inFIGS. 16A and 16B are the same as the respective orientations of theX-axis, the Y-axis and the Z-axis in FIG. 13. To keep the drawingsimple, the semiconductor laser element 1, the diffraction grating 2,and the information recording medium 6 are the only structural elementsshown in this drawing. The recording face of the information recordingmedium 6 is tilted with respect to the horizontal plane (indicated by adashed line) by an angle δ around the X-axis. In FIG. 16, points A and Bdenote points of the laser light irradiated onto the informationrecording medium 6, point C denotes the point of emission of the laserlight, and point D denotes a point of light returning from theinformation recording medium 6 on the end face of the semiconductorlaser 1.

[0016] As is shown in FIG. 16A, the light that is emitted from point Cof the semiconductor laser element 1 is reflected at point B on theinformation recording medium 6, diffracted at the diffraction grating 2,and returns to point D at the end face of the semiconductor laserelement 1. There, the light is reflected, passes the diffraction grating2 again, and reaches point A at the information recording medium 6(resulting in the light path 1: C→B→D→A). Reflected several times inthis manner, the light may reach a photodetector group 7 (not shown inthis drawing). As a result, a phase difference results, caused by thedifference of the light path length of the light beam reaching thephotodetector group 7 following the original path, and the two lightbeams may interfere with each other.

[0017] As is shown in FIG. 16B, the light emitted from the emissionpoint C of the semiconductor laser element 1 is reflected at point A onthe information recording medium 6, passes the diffraction grating 2,and returns to point D at the end face of the semiconductor laserelement 1. There, the light is reflected, passes the diffraction grating2 again, and reaches point A at the information recording medium 6(resulting in the light path 2: C→A→D→A). Also in this case, the light,which has been reflected several times in this manner, may reach thephotodetector group 7 (not shown in this drawing). As a result, a phasedifference results, caused by the difference of the light path length ofthe light beam reaching the photodetector group 7 following the originalpath, and the two light beams may interfere with each other.

[0018] The degree of this interference (i.e. the interference strength)changes with the phase difference, which depends on the tilt angle δ inthe track direction of the information recording medium 6. Consequently,there is the problem that the signal strength based on the sub-beamspots varies, and there is an offset in the tracking error signal.

[0019] Similarly, interferences due to multiple reflections between themain beam and the sub-beams may occur, and lead to the problem that notonly the S/N ratio of the reproduction signal but also the S/N ratio ofthe tracking error signal decreases.

SUMMARY OF THE INVENTION

[0020] It is an object of the present invention to provide an opticalelement, wherein the light amount of both the main beam and thesub-beams can be increased without increasing the light emission of thesemiconductor laser element 1, and the S/N ratio of the main beam andthe sub-beams can be enhanced.

[0021] It is another object of the present invention to provide anoptical device, wherein an offset of the tracking error signal isreduced by suppressing multiple reflections between the informationrecording medium and the optical components used for the optical device,and the S/N ratio of the reproduction signal or the tracking errorsignal is increased by suppressing interference between the main beamand sub-beams.

[0022] An optical device in accordance with the present inventioncomprises:

[0023] a light-emitting element;

[0024] a diffraction grating for splitting light emitted from saidlight-emitting element into a plurality of beams, the diffractiongrating comprising a first grating region and a second grating regionwith a diffraction efficiency that is different from a diffractionefficiency of the first grating region; and

[0025] a focussing member for focussing light that has passed throughthe diffraction grating.

[0026] In the diffraction grating in the optical device of the presentinvention, the diffraction efficiency of the first grating region fortransmitting the main beam is independent from the diffractionefficiency of the second grating region for transmitting a sub-beam,which increases the light utilization efficiency.

[0027] Moreover, by setting the diffraction efficiency of the first andthe second grating region such that the diffraction efficiency of±1-order diffraction light of the first grating region for transmittingthe main beam is lower than the diffraction efficiency of ±1-orderdiffraction light of the second grating region for transmitting thesub-beams, the diffraction grating in the optical device of the presentinvention can suppress multiple reflections between the informationrecording medium and the optical components used in the optical device.As a result, the offset of the tracking error signal is reduced.Moreover, interference between the main beam and the sub-beams can besuppressed. The synergy of this effect and the above-noted effect ofincreasing the light utilization efficiency enhances the S/N ratio ofthe reproduction signal and the tracking error signal even more.

BRIEF DESCRIPTION OF THE DRAWINGS

[0028]FIGS. 1A and 1B show the diffraction gratings of optical devicesin accordance with a first embodiment of the present invention.

[0029]FIG. 2A is a top view of another example of a diffraction gratingof an optical device in accordance with the first embodiment of thepresent invention. FIGS. 2B-D are cross-sections taken along I-I in FIG.2A viewed in the arrow direction.

[0030]FIGS. 3A and 3B show other examples of the diffraction gratings ofoptical devices in accordance with the first embodiment of the presentinvention.

[0031]FIG. 4 shows an optical device in accordance with the firstembodiment of the present invention.

[0032]FIG. 5 shows an optical device in accordance with the firstembodiment of the present invention.

[0033]FIG. 6 shows an optical device in accordance with the firstembodiment of the present invention.

[0034]FIG. 7 shows an optical device in accordance with the firstembodiment of the present invention.

[0035]FIG. 8 shows an optical device in accordance with the firstembodiment of the present invention.

[0036]FIG. 9 shows an optical device in accordance with the firstembodiment of the present invention.

[0037]FIG. 10 shows an optical device in accordance with the firstembodiment of the present invention.

[0038]FIG. 11 shows an optical device in accordance with the firstembodiment of the present invention.

[0039]FIG. 12 shows the diffraction gratings of optical devices inaccordance with a second embodiment of the present invention.

[0040]FIG. 13 shows an optical device in accordance with the presentinvention and the prior art.

[0041]FIG. 14 shows a diffraction grating in an optical device of theprior art.

[0042]FIG. 15 shows the diffraction efficiency as a function of thegrating depth in a diffraction grating of the prior art.

[0043]FIG. 16 illustrates multiple reflections between an informationrecording medium and the optical device.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0044] The following is a description of the preferred embodiments ofthe present invention, with reference to FIGS. 1 to 12.

[0045] First Embodiment

[0046] The following explains an optical device in a first embodiment ofthe present invention.

[0047] The configuration of the optical device in the first embodimentof the present invention is basically the same as that of theconventional optical device illustrated in FIG. 13, but the structure ofthe diffraction grating 2 in the first embodiment of the presentinvention differs from the conventional one in the following aspects.

[0048]FIG. 1A is a top view of the diffraction grating 2 of the opticaldevice in the first embodiment of the present invention. Theorientations of the X-axis, the Y-axis and the Z-axis in FIG. 1 are thesame as the respective orientations of the X-axis, the Y-axis and theZ-axis in FIG. 13. In FIG. 1A, the diffraction grating 2 has astripe-shaped first grating region 8 parallel to the X-axis, and secondgrating regions 9 formed on both sides of the first grating region 8.The diffraction efficiency of the first grating region 8 is differentfrom that of the second grating regions 9. The grating in the firstgrating region 8 and the grating in the second grating region 9 areformed parallel to the stripe-shaped first grating region 8, that is,parallel to the X-axis. The pitch of the grating in the first gratingregion 8 is the same as the pitch of the grating in the second gratingregions 9.

[0049] The zero-order diffraction light in the region 11 in FIG. 1Aenters the collimator lens 4, the +1-order diffraction light in theregion 12 enters the collimator lens 4, and the −1-order diffractionlight in the region 13 enters the collimator lens 4.

[0050] The grating element 2 is arranged so that the optical axis of thelaser light emitted from the semiconductor laser element 1 in FIG. 13passes through a center portion of the first grating region 8, so thatthe zero-order diffraction light passing through the first gratingregion 8 is used as the main beam, and the ±1-order diffraction lightpassing through the second grating regions 9 is used as the sub-beams.Thus, the diffraction grating 2 of the optical device in the firstembodiment is divided into a first grating region 8 for generating amain beam and second grating regions 9 for generating sub-beams, so thatby individually adjusting the grating depth in these different regions,the diffraction efficiency for the zero-order diffraction light and the±1-order diffraction light can be set individually. Therefore, thediffraction efficiency of the zero-order diffraction light in the firstgrating region 8 can be raised without influencing the intensity of the±1-order diffraction light generated in the second grating regions 9,and conversely, the diffraction efficiency of the ±1-order diffractionlight in the second grating region 9 can be raised without influencingthe intensity of the zero-order diffraction light generated in the firstgrating regions 8.

[0051] For this reason, with the optical device in the first embodiment,in order to increase the intensity of the zero-order diffraction lightgenerated in the first grating region 8, and to increase the intensityof the ±1-order diffraction light generated in the second gratingregions 9, the diffraction efficiency of the zero-order light in thefirst grating region 8 is set to be larger than the diffractionefficiency of the zero-order diffraction light in the second gratingregions 9. Thus, the light utilization efficiency can be increased.Moreover, the light amount of both the main beam and the sub-beams canbe increased and their S/N-ratio can be raised, without increasing theemission intensity of the semiconductor laser element 1 itself. Thismeans that the ±1-order diffraction efficiency is decreased, because thezero-order diffraction efficiency of the first grating region 8 isincreased. Consequently, influences due to multiple reflections alonglight path 1 (C→B→D→A) in FIG. 16A and light path 2 (C→A→D→A) in FIG.16B are suppressed, so that interferences between the main beam and thesub-beam are diminished. Consequently, the synergy of this effect andthe above-noted effect of increasing the light utilization efficiencyenhances the S/N ratio of the reproduction signal and the tracking errorsignal. And for the same reason, an offset of the tracking error signalcan be decreased.

[0052] The pitch of the grating in the first grating region 8 can be thesame as the pitch in the second grating regions 9, or it can bedifferent.

[0053] The first grating region 8 and the second grating region 9 alsocan be configured as shown in FIG. 2. FIG. 2A is a top view of thediffraction grating 2. FIGS. 2B-D are cross-sectional views in the arrowdirection taken along I-I in FIG. 2A. In FIGS. 2B-D, the dotted linesindicate that the same shape repeats over and over.

[0054] The depth of the diffraction grating in the first grating region8 can be different from the depth of the diffraction grating in thesecond grating region 9. For example, if the depth of the diffractiongrating in the first grating region 8 is made smaller than the depth ofthe diffraction grating in the second grating region 9 as shown in FIG.2B, then the diffraction efficiency of the zero-order diffraction lightin the first grating region 8 is set to be larger than the diffractionefficiency of the zero-order diffraction light in the second gratingregion 9.

[0055] Moreover, a stepwise variation of the depth of the diffractiongrating with a constant period in the first grating region 8 or thesecond grating region 9 is also possible. For example, if the depth ofthe diffraction gratings in the first grating region 8 varies stepwise,and the depth of the diffraction grating in the first grating region 8is made smaller than the depth of the diffraction grating in the secondgrating region 9 as shown in FIG. 2C, then the diffraction efficiency ofthe zero-order diffraction light in the first grating region 8 is set tobe larger than the diffraction efficiency of the zero-order diffractionlight in the second grating region 9, which improves the lightutilization efficiency. Furthermore, with a step-wise approximatedblaze-shape, the occurrence of 31 1-order diffraction light can besuppressed. That is to say, the zero-order diffraction efficiency of thegrating region 8 is enhanced even further, which not only increases thelight utilization efficiency, but also improves the S/N ratio of thereproduction signal and the tracking error signal, because interferencesbetween the main beam and the sub-beams are suppressed.

[0056] Moreover, if the diffraction grating in the first grating region8 is a blazed grating (saw-tooth-shaped) as shown in FIG. 2D, then theS/N ratio of the reproduction signal and the tracking error signal isimproved even further, because the occurrence of −1-order diffractionlight can be suppressed completely.

[0057] Furthermore, the diffraction grating pattern of the first gratingregion 8 also can be different from the diffraction grating pattern ofthe second grating region 9. For example, if the diffraction grating inthe first grating region 8 is curved as shown in FIG. 3A, then the lightof the main beam returning from the information recording medium 6 canbe focussed and diffracted to be guided towards a light-receivingelement, and the reproduction signal can be detected. Moreover, thefirst grating region 8 also can be composed of a plurality of regionswith diffraction gratings having different grating directions as shownin FIG. 3B.

[0058] It is also possible to make the first grating region 8stripe-shaped, and tilt the direction of the diffraction grating of thesecond grating region 9 by a predetermined angle against the directionof the diffraction pitch of the first grating region 8. By doing so, thediffraction efficiency of the zero-order diffraction light in the firstgrating region 8 can be set to be larger than the diffraction efficiencyof the zero-order diffraction light in the second grating region 9, andif, for example, a tracking error signal is detected with the three-beammethod, then it becomes easy to realize an arrangement wherein the focusspots of the main beam and the sub-beams are separated by a distance ofonly ¼ the track pitch on the information recording medium 6.

[0059] This embodiment has been explained for the case that the region12 and the region 13 overlap with the first grating region 8, as shownin FIG. 1A. However, if the optical system is designed so that theregion 12 and the region 13 are formed completely inside the secondgrating regions 9, as shown in FIG. 1B, then the entire sub-beams passthrough the second grating regions 9. Consequently, the intensity of thesub-beams is completely independent of the diffraction efficiency of thefirst grating region 8, so that the diffraction efficiency of the firstgrating region 8 can be set freely, without giving consideration to theintensity of the sub-beams.

[0060] Moreover, if the width of the first grating region 8 in theY-axis direction is at least 2d tan(sin⁻¹(NA)), wherein d is thedistance from the emission plane of the semiconductor laser element 1 tothe diffraction grating 2, and NA is the numerical aperture of thecollimator lens 4, then only the zero-order diffraction light passingthrough the first grating region 8 becomes the main beam. Consequently,the intensity of the main beam becomes completely independent of thediffraction efficiency of the second grating region 9, so that thediffraction efficiency of the first grating region 8 can be set freelywithout giving consideration to the intensity of the sub-beams.

[0061] This embodiment has been explained for the case of an opticaldevice having an infinite optical system with a collimator lens 4 and anobjective lens 5. However, the present invention can be applied equallyto a finite optical system using only the objective lens 5.

[0062] If the diffraction grating 2 and the holographic element 3 areintegrated into one component, as shown in FIG. 4, then the number ofoptical components can be reduced, and the optical device can be madesmaller and thinner.

[0063] If the semiconductor laser element 1 and the photodetector groups7 are arranged in the same package 14, and this package 14 is sealedwith an optical component, into which the diffraction grating 2 and theholographic element 3 are integrated, as shown in FIG. 5, then theoptical system can be made smaller and thinner, while increasing thereliability of the optical device considerably.

[0064] Moreover, integrating the semiconductor laser element 1 and thephotodetector groups 7 into one component on a silicon substrate 15, asshown in FIG. 6, makes the assembly easier than if separate elements arearranged inside the package 14. Moreover, using semiconductormicroprocessing technology, it is possible to integrate on substrate 15a circuit for current-voltage conversion and evaluation of the electricsignals from the photodetector groups 7. This makes it possible toreduce noise caused by the wiring inside the optical device, so that anoptical device with a better S/N ratio can be realized. This integrationcan be realized as a hybrid structure by bonding the chip to thesemiconductor laser element 1, after forming all the photodetectorgroups 7 by semiconductor micro-processing technology on a siliconsubstrate 15. If the semiconductor laser element 1 is of thesurface-emitting type, the emitting surface should be faced upward andchip-bonded. If the semiconductor laser element 1 is of the endface-emitting type, it is possible to form a concave portion in thesubstrate 15 by semiconductor micro-processing technology, as shown inFIG. 7, and to chip-bond the semiconductor laser element 1 into thisportion.

[0065] If a micro-mirror 17 is formed by forming a face slant at anangle of 45° in this concave portion, and depositing a metal or adielectric film on this face, then the light emitted from thesemiconductor laser element 1 is reflected from the micro-mirror 17, sothat the light can be guided in an upward direction.

[0066] And using semiconductor micro-processing technology, it is alsopossible to form a monitoring photodetector 16 for receiving the lightemitted by the semiconductor laser element 1 from the side that isopposite to the micro-mirror 17, and adjusting the output of thesemiconductor laser element 1. This configuration has the effect thatthe optical output of the semiconductor laser element 1 can be adjustedconstantly to the optimal output, and waste of electric power due toexcessive optical output can be avoided.

[0067] On the other hand, it is also possible to use semiconductorhetero-epitaxy to form a compound semiconductor layer (not shown in thedrawings) monolithically on the silicon substrate 15, and to form thesemiconductor laser element 1 and the photodetector groups 7 on thesilicon substrate 15 or on the compound semiconductor layer. In thiscase, the semiconductor laser element 1 and the photodetector groups 7can be integrated into one component without using the silicon substrate15, and using only the compound semiconductor layer.

[0068] Moreover, the present embodiment has been explained for anoptical device in which both the +1-order diffraction light and the−1-order diffraction light of the holographic element 3 are used todetect the reproduction signal and the various servo signals. However,if only the +1-order diffraction light or the −1-order diffraction lightof the holographic element 3 is used to detect the reproduction signaland the various servo signals as shown in FIG. 8, the number ofphotodetectors constituting the photodetector groups 7 can be reduced,which makes the optical device cheaper.

[0069] Adding, as shown in FIG. 9, a polarization beam splitter 18, areflector 19, a polarization separator 20 and a photodetector group 21for polarization signal detection to the configuration of the opticaldevice of FIG. 1 makes optomagnetic signal detection possible. In FIG.9, the light that is reflected from the information recording medium 6and incident on the polarization beam splitter 18 is split up into abeam that travels towards the holographic element 3 and a beam thattravels towards the reflector 19. The reflected light that has beensplit by the polarization beam splitter 18 and travels towards theholographic element 3 is diffracted and focussed by the holographicelement 3 onto the photodetector group 7, and is used to calculateand/or detect the servo signal, as explained above.

[0070] On the other hand, the returning light that is split by thepolarization beam splitter 18 and travels toward the reflector 19 isreflected by the reflector 19, polarized and separated into P-polarizedlight and S-polarized light by the polarization separator 20, and guidedto the photodetector group 21 for polarization signal detection,whereupon the reproduction signal is calculated. Thus, the lightutilization efficiency of optical devices for optomagnetic signaldetection, which are often said to have an inferior light utilizationefficiency, can be improved, which enables operation with asemiconductor laser element with low output power. Furthermore, byenhancing the S/N ratio of the reproduction signal, a high-qualityreproduction signal can be attained. That is to say, the diffractionefficiency of the ±1-order diffraction light is reduced, because thediffraction efficiency of the zero-order diffraction light of the firstgrating region 8 is increased. Consequently, since the influence ofmultiple reflection is suppressed, interference effects between the mainbeam and the sub-beams can be decreased. Therefore, the synergy of thiseffect and the effect of increasing the light utilization efficiencyenhances the S/N ratio of the reproduction signal and the tracking errorsignal. For the same reason, an offset of the tracking error signal isalso reduced.

[0071] Moreover, if the polarization beam splitter 18, the reflector 19,and the polarization separator 20 are integrated into one component, asshown in FIG. 10, the number of optical components can be reduced, sothat the optical device can be made smaller, thinner and cheaper.

[0072] If the semiconductor laser element 1, the photodetector groups 7and the photodetector group 21 for polarization signal detection areintegrated on a substrate 15, arranged in a package 14, and this package14 is sealed with an optical component, into which the diffractiongrating 2 and the holographic element 3 are integrated, and thepolarization beam splitter 18, the reflector 19, and the polarizationseparator 20 are integrated into one component and mounted on theoptical component into which the diffraction grating 2 and theholographic element 3 have been integrated, as shown in FIG. 11, thenthe optical device can be made smaller and thinner, while increasing thereliability of the optical device considerably.

[0073] The above-noted application examples relate only to an opticalsystem for reproduction/recording on an information recording medium,but it should be understood that the optical device of the presentinvention can be applied equally to other optical information processingsystems.

[0074] Second Embodiment

[0075] The following explains an optical device in a second embodimentof the present invention.

[0076] The configuration of the optical device in the second embodimentof the present invention is basically the same as that in the firstembodiment, but the structure of the diffraction grating 2 in the secondembodiment of the present invention differs from the one in the firstembodiment in the following aspects.

[0077]FIG. 12 is a top view of the diffraction grating 2 of the opticaldevice in the second embodiment of the present invention. Theorientations of the X-axis, the Y-axis and the Z-axis in FIG. 12 are thesame as the respective orientations of the X-axis, the Y-axis and theZ-axis in FIG. 13. In FIG. 12, gratings parallel to the X-axis andhaving a constant pitch are formed for the diffraction grating 2.However, in the center portion of the diffraction grating 2, there is anon-grating region 10 without grating, which is stripe-shaped and formedparallel to the X-axis.

[0078] The location of the non-grating region 10 corresponds to thelocation where the first grating region 8 is formed in the diffractiongrating 2 in the first embodiment. As has been explained for the firstembodiment, the zero-order diffraction light passing the firstdiffraction region 8 in the diffraction grating 2 is used as the mainbeam, and to increase the intensity of the main beam, it is preferablethat the diffraction efficiency of the zero-order diffraction light inthe first grating region 8 is set to 100%. Setting the diffractionefficiency of the zero-order diffraction light in the first gratingregion 8 to 100% is equivalent to forming no grating at all in the firstgrating region 8 in the first embodiment, that is, providing thenon-grating region of the second embodiment.

[0079] Thus, by forming a non-grating region in the center portion ofthe diffraction grating 2, it is possible to increase the intensity ofthe main beam optimally. And what is more, ±1-order diffraction light isnot generated, because the first grating region 8 is a non-gratingregion. Consequently, it is possible to completely eradicate the noisecomponents due to interferences between the main beam and the sub-beamscaused by multiple reflections along light path 1 (C→B→D→A) in FIG. 16Aand light path 2 (C→A→D→A) in FIG. 16B. Therefore, the S/N ratio of thereproduction signal and the tracking error signal can be enhancedconsiderably. Moreover, the offset of the tracking error signal can bedecreased.

[0080] The various application examples explained in the firstembodiment also can be used for the second embodiment. Also, the opticaldevice in the second embodiment can be used for optical informationprocessing systems other than systems for reproduction/recording of aninformation recording medium.

[0081] Moreover, the grating of the diffraction grating 2 can be tiltedby a predetermined angle with respect to the stripe-shaped non-gratingregion 10.

[0082] Thus, providing the diffraction grating in the optical device ofthe present invention, which splits the light emitted from asemiconductor laser element into a plurality of beams, with a pluralityof diffraction grating regions with different functions, the lightutilization efficiency of the optical device can be increased vastly, sothat the light amount of both the main beam and the sub-beams can beincreased without increasing the light emission of the semiconductorlaser element, and the S/N ratio of the main beam and the sub-beams canbe enhanced.

[0083] Moreover, by making the ±1-order diffraction efficiency of thefirst grating region for transmitting the main beam in the opticaldevice of the present invention lower than the ±1-order diffractionefficiency of the second grating region, multiple reflections betweenthe information recording medium and the semiconductor laser element canbe suppressed, so that the offset of the tracking error signal isreduced.

[0084] The invention may be embodied in other specific forms withoutdeparting from the spirit or essential characteristics thereof. Theembodiments disclosed in this application are to be considered in allrespects as illustrative and not restrictive, the scope of the inventionbeing indicated by the appended claims rather than by the foregoingdescription, and all changes that come within the meaning and range ofequivalency of the claims are intended to be embraced therein.

What is claimed is:
 1. An optical device comprising: a light-emittingelement; a diffraction grating for splitting light emitted from saidlight-emitting element into a plurality of beams, the diffractiongrating comprising a first grating region and a second grating regionwith a diffraction efficiency that is different from a diffractionefficiency of the first grating region; and a focussing member forfocussing light that has passed through the diffraction grating.
 2. Anoptical device comprising: a light-emitting element for irradiatinglight onto an information recording medium; a diffraction grating forsplitting light emitted from said light-emitting element into aplurality of beams, the diffraction grating comprising a first gratingregion and a second grating region with a diffraction efficiency that isdifferent from a diffraction efficiency of the first grating region; afocussing member for focussing the beams that have been split by thediffraction grating on the information recording medium; a deflectionmember for deflecting the plurality of beams after they have beenreflected from the information recording medium; and a photodetector forreceiving the plurality of beams after they have been deflected by thedeflection member.
 3. The optical device of claim 1 or 2, wherein adiffraction grating depth in the first grating region is different froma diffraction grating depth in the second grating region.
 4. The opticaldevice of claim 1 or 2, wherein a diffraction grating depth in the firstgrating region and/or a diffraction grating depth in the second gratingregion changes stepwise with a constant period.
 5. The optical device ofclaim 1 or 2, wherein a diffraction grating in the first grating regionand/or a diffraction grating in the second grating region is a blazedgrating.
 6. The optical device of claim 1 or 2, wherein the diffractiongrating pattern in the first grating region is different from thediffraction grating pattern in the second grating region.
 7. The opticaldevice of claim 1 or 2, wherein a diffraction efficiency of zero-orderdiffraction light in the first grating region is larger than adiffraction efficiency of zero-order diffraction light in the secondgrating region.
 8. The optical device of claim 2, wherein the zero-orderdiffraction light in the first grating region is used as a main beam forrecording/reproducing an information signal on/from the informationrecording medium; +1-order and/or −1-order diffraction light in thesecond grating region is used as a sub-beam for detecting a trackingerror signal; and the entire sub-beam passes through the second gratingregion.
 9. The optical device of claim 1 or 2, wherein the first gratingregion is stripe-shaped; and the grating in the first grating region andthe grating in the second grating region are parallel to the firstgrating region.
 10. The optical device of claim 1 or 2, wherein thefirst grating region is stripe-shaped; and the grating in the firstgrating region and/or the grating in the second grating region is tiltedby a predetermined angle with respect to the first grating region. 11.An optical device, comprising: a light-emitting element; a diffractiongrating for splitting light emitted from said light-emitting elementinto a plurality of beams, the diffraction grating comprising anon-grating region; and a focussing member for focussing light that haspassed through the diffraction grating; wherein all or a portion of thelight that has passed through the non-grating region enters thefocussing member.
 12. An optical device comprising: a light-emittingelement for irradiating light onto an information recording medium; adiffraction grating for splitting light emitted from said light-emittingelement into a plurality of beams, the diffraction grating comprising anon-grating region; a focussing member for focussing the beams that havebeen split by the diffraction grating onto the information recordingmedium; a deflection member for deflecting the plurality of beams afterthey have been reflected from the information recording medium; and aphotodetector for receiving the plurality of beams after they have beendeflected by the deflection member; wherein all or a portion of thelight that has passed through the non-grating region enters thefocussing member.
 13. The optical device of claim 11 or 12, wherein adiffraction grating depth of the diffraction grating changes stepwisewith a constant period.
 14. The optical device of claim 11 or 12,wherein the diffraction grating is a blazed grating.
 15. The opticaldevice of claim 11 or 12, wherein a diffraction efficiency of thediffracting grating is adjusted so as to maximize the amount of lightthat enters the focussing member after having passed through a region ofthe diffraction grating other than the non-grating region.
 16. Theoptical device of claim 12, wherein light that has passed through thenon-grating region is used as a main beam for recording/reproducing aninformation signal on/from the information recording medium; +1-orderand/or −1-order diffraction light in the diffraction grating is used asa sub-beam for detecting a tracking error signal; and the entiresub-beam passes through a region of the diffraction grating other thanthe non-grating region.
 17. The optical device of claim 11 or 12,wherein the non-grating region is stripe-shaped; and the grating of thediffraction grating is parallel to the non-grating region.
 18. Theoptical device of claim 11 or 12, wherein the non-grating region isstripe-shaped; and the grating of the diffraction grating is tilted by apredetermined angle with respect to the non-grating region.
 19. Theoptical device of claim 2 or 12, further comprising a circuit foramplifying an electrical signal from the photodetector.
 20. The opticaldevice of claim 2 or 12, wherein the deflection member is a diffractiongrating functioning as a lens.
 21. The optical device of claim 2 or 12,further comprising a polarization beam splitter
 22. The optical deviceof claim 2 or 12, wherein at least the light-emitting element and thephotodetector are integrated in the same housing.